1. Field of the Invention
The present invention relates generally to improvements in electronic filters, and, more particularly, but not by way of limitation to improvements in electronic filters suitable for use in a disc drive PRML read channels.
2. Brief Description of the Prior Art
In disc drives used to store computer files, the files are stored along concentric data tracks defined in magnetizable coatings on the surfaces of rotating discs. To this end, write heads are positioned adjacent the disc surfaces and are radially movable to align with a track selected to store a file so that the track can be magnetized by passing a current through the write head. The direction in which the data track is magnetized is determined by the direction of the current through the write head so that a file can be stored by magnetizing the track in a pattern that is derived from the file. More particularly, the write head current is supplied by a write driver to which a sequence of bits derived from the file is clocked and the direction of the current in each clock period is determined by the logic values of bits received by the write driver. Consequently, successive segments of the data track will be magnetized in a pattern that reflects the contents of the file. The magnetized track segments, or data elements, in turn, produce a magnetic field that can be sensed by a read head to generate an electrical signal that varies with time in a way that reflects the sequence of data elements along the data track to permit retrieval of the file.
In a conventional disc drive, the write driver reverses the direction of the current through the write head each time a logical 1 occurs in the sequence of bits received by the write driver to, in turn, reverse the direction of magnetization of the magnetic medium along the track each time a logical 1 occurs in the sequence. During subsequent reading, each reversal in the magnetization of the data track, a so-called flux transition, gives rise to a peak in the signal generated by the read head and peak detection circuitry is used to place the peaks within "read windows" established by a phase locked loop that receives signals from the peak detection circuitry to establish a read clock that is synchronized with the passage of flux transitions by the read head. More particularly, the occurrence of a signal peak within a read window is an indicator of a flux transition along the data track. Consequently, since the flux transitions are generated by logical 1's in the encoded user file, the occurrence of a signal peak indicates a logical 1 in the sequence of data bits that were received by the write driver. The read phase locked loop is then used to clock logical 1's for those read windows in which peaks are detected, and logical 0's for those read windows in which peaks are not detected, to circuitry which regenerates the stored file.
While conventional disc drives which make use of the above scheme are able to operate reliably to store large quantities of data, it has become increasingly difficult to raise the data storage capacity of disc drives of this type to higher and higher levels that are demanded by users of disc drives. More particularly, the data storage capacity of a disc drive depends upon the transfer rate of data bits between the write head and a data track and between the data track and a read head and problems have arisen with increasing the transfer rates of drives of the generic type.
Several effects tend to limit transfer rate in conventional disc drives. The synchronization between the read clock and the passage of flux transitions by the read head depends upon the correspondence between peaks in the signal induced in the read head and passage of individual flux transitions by the read head. However, the magnetic field from which the read head signal is derived is a superposition of the magnetic fields produced by all of the flux transitions on the disc. Consequently, as the transfer rate is increased, to decrease the spacing of flux transitions along a data track, so-called "intersymbol interference"; that is, significant superposition of magnetic fields from successive flux transitions on a data track, causes peaks in the read head to be shifted in time from the times that such peaks would occur for an isolated flux transition. While the effects of peak shift can be minimized; for example, by pulse slimming, encoding of user data and prewrite compensation, it remains a problem with peak detect disc drives that limits the transfer rate of data in such a disc drive.
Moreover, the superposition of magnetic fields of successive flux transitions gives rise to a second problem. The fields produced by adjacent flux transitions superpose destructively so that the magnitude of the signal induced in a read head by passage of a flux transition decreases with increasing transfer rate. Consequently, the signal to noise ratio of the output of the read head decreases with increasing transfer rate to increase the number of errors that occur during the reading of data. While, as in the case of peak shift, corrective measures; for example, adaptive signal filtering and the use of error detection and correction circuitry, can be taken, it becomes increasingly difficult to employ these measures as the transfer rate is increased. The net result is that, while room for improvement of disc drives that make use of peak detection circuitry may still exist, the improvements are achieved only by measures that are becoming increasingly difficult and expensive to employ.
Because of this difficulty in increasing the data transfer rate in disc drives employing peak detection in the read channel, practitioners have, in recent years, turned to the use of so-called PRML read channels in disc drives. In disc drives of this type, partial response signaling is utilized to control, rather than to suppress, intersymbol interference and the effect of noise is minimized by the use of maximum likelihood detection of the magnetization of sequences of segments of the data track. To this end, signals corresponding to individual flux transitions are filtered to a signal which, in the absence of noise, would have a nominal form and the signals are then sampled at times determined in relation to this nonfinal form for maximum likelihood detection in which each bit of encoded data is recovered in the context of the sequence of bits that were written to the disc to limit the effect of noise.
The use of partial response signaling and maximum likelihood detection in a disc drive places especially stringent requirements on filtering of the signal induced in the read head prior to maximum likelihood detection. While maximum likelihood detection limits the effect of random noise in the reading of a block of data from a disc, variances between the nominal form to which signals induced in the read head are ideally filtered and the actual filtered signal constitute systematic noise which can generate errors in the data that is eventually recovered from a data track. Consequently, electronic filtering of the signal induced in the read head plays an important role in the operation of a disc drive that makes use of PRML.
A complicating factor in the filtering of the signal induced in a disc drive read head to a nominal form is that the form of the signal induced in the read head, even disregarding noise, varies with the radius of the data track upon which data being read is stored. Such variation, which is related to the increase of track circumference with track radius, arises from the use of the magnetic interaction between the read head and the magnetization of data track that enables the data track to be read and is, consequently, an inherent characteristic of signals induced in a read head. Thus, the filter in a PRML read channel must not only be capable of filtering the signal induced in a read head to a reasonable approximation of a specific wave form but must be able to do so adaptively in relation to track radius.
In the past, the favored approach to effecting the adaptive filtering required in a disc drive that employs partial response signaling and maximum likelihood detection has been to employ digital electronics in the construction of the filter. More particularly, the filter contains a digital to analog converter that digitizes the signal after partial filtering has taken place and a transversal equalizer that employs registers as delay elements so that filtering can be effected by adding successive sample values with appropriate weighting of the sample values. However, the use of digital electronics in the filter gives rise to another problem. Since clocked registers are used as delay elements, a delay is introduced between the times at which the signal is sampled and the time the filtered signal is received by circuitry used to time the taking of samples and the shifting of the registers in the filter. Further, since the operation of such circuitry is based on variances between actual sample values and sample values that are determined for specific sample moments referred to the nominal wave form, the delay introduced by the filter limits the extent to which a read clock, used to effect sampling and shifting of the registers in the equalizer, can be synchronized with the passage of flux transitions by the read head to cause the samples to be taken at sampling moments defined in relation to the nominal wave form. As a result, while the extent to which the filtered signal approximates the nominal wave form can be increased by increasing the number of delay elements, the added delay in the generation of the filtered signal that arises from the increase in delay elements increases errors in the times at which the wave form is sampled. Consequently, the use of digital filtering requires that compromises be effected between the extent to which the filtered signal approximates the nominal wave form and the appropriateness of the moments at which the filtered signal is sampled.